By shortening the length of the video or increasing the size of the disc
Bitrate is given in BIT PER SECOND, it is obtained by dividing the numbers of bits you have on the disc by the number of second (of video) you want to put on it.
What you want to put into these bits (full frame picture, lettebox, overscanned or not...) does not enter in the equation.
(what you determined in CQMatic by test sampling is not the AVG bitrate but the quality of the compression, ie the CQ).
Note: is there any virus on this forum that make any people coming nut ?